Rod and cone photoreceptors of vertebrates retinas hyperpolarize in response to light. Rods are very sensitive, but slow to respond. Cones are insensitive but quick. Phototransduction in both rods and cones occurs via G-protein mediated light stimulated cyclic GMP hydrolysis. Many phototransduction proteins, including opsin, transducin and phosphodiesterase catalytic and inhibitory subunits, are products of different genes in rods and cones. The first aim of this proposal is to identify kinetic differences between rod and cone phototransduction enzymes and to explain, at a molecular level, the physiological differrnces between rods and cones. Cone photorecpetors are to be isolated, cone phototransduction enzymes individually purified and directly compared biochemically with their rod counterparts. Recoverin is a recently identified Ca4++-dependent regulator of photoreceptor guanylate cyclase. As part of the comparison of rod and cone phototransduction enzymes, recoverin is to be characterized and the existence of rod and cone-specific recoverins is to be investigated. THe second aim of this proposal is to determine physiological consequences of expressing normal and mutant forms of phototransduction enzymes in rod photoreceptors. Several lines of transgenic mice carrying transducin transgenes have been produced. Retinas from these mice are to be characterized biochemically and electrophysiologically to determine how altered transducin activity affects the photoresponse. Other phototransduction enzymes are also to be expressed as potential dominant mutations in these proteins are identified. The overall aim of this proposal is to identify factors that determine the speed and sensitivity of the photoresponse.